1. Field of the Invention
The present invention is related to a carbon-doped alumina coating which has application as a MEMS (Micro-Electro-Mechanical Systems) surface protective, functional film.
2. Brief Description of the Background Art
This section describes background subject matter related to the invention, with the purpose of aiding one skilled in the art to better understand the disclosure of the invention. There is no intention, either express or implied, that the background art discussed in this section legally constitutes prior art.
Protective coatings currently used in the manufacture of MEMS devices include but are not limited to: moisture barrier coatings, oxidation barriers, anti-stiction coatings, “release” coatings, protective coatings for microdevices such as microfluidic devices, ink-jet heads, thin film heads, and other electronic and optical devices.
Currently known fluorocarbon coatings, such as self-assembled monolayers (SAMs) are used to provide a hydrophobic surface function; however, these coatings do not offer sufficient wear resistance. This is particularly true with respect to micromechanical or microelectromechanical devices, in which a mechanical contact (sliding, touching, or physical interaction between the parts) requires durable, protective and non-sticky (non-tacky) films.
Wear-resistant, low surface energy coatings of silicon carbide (SiC) can be deposited by a chemical vapor deposition (CVD) method, providing considerable degree of protection, specifically wear reduction. W. Ashurst et al. in an article entitled “Tribological impact of SiC encapsulation of released polycrystalline silicon microstructures”, Triboloby Letters, v. 17, 2004, 195-198, describe a method for coating released polysilicon microstructures with a thin, conformal coating of SiC derived from a single source precursor. The precursor was 1,3-disilabutane (DSB). This coating method has been successfully applied to micromechanical test devices which allow the evaluation of friction and wear properties of the coating. Data for the coefficient of static friction of the SiC coatings produced from DSB is presented in FIG. 1 of this application, for reference purposes. FIG. 1 shows a graph 100 which illustrates the coefficient of friction, μs, on axis 104, as a function of the number of wear cycles in millions on axis 102. The wear testing was done using a polysilicon sidewall friction tester of the kind described by W. R. Ashurst et al. in Tribology Lett. 17 (2004)195-198. Curve 106 represents wear testing of an oxidized polysilicon substrate with a native oxide surface. Curve 108 represents wear testing of an anti-adhesion coating produced from vapor deposited DDMS over the surface of the sidewall friction tester. Curve 110 represents wear testing of a silicon substrate which was treated with an oxygen plasma, followed by deposition of an SiC coating. The SiC coating was deposited by plasma assisted low pressure CVD, from a SiCl4 precursor at about 800° C., using a technique generally known in the art. The wear was examined using scanning electron microscopy (SEM) on devices which were cycled repetitively under a nominal load. This testing shows that the application of an SiC coating having a thickness of about 40-50 nm provides good wear resistance as well as a significant reduction in friction on a micro scale.
A wear-resistant low surface energy coating which can be produced at low temperatures (in the range of about 200° C. or less) would be highly desirable. Such a coating can be produced using atomic layer deposition (ALD) films produced at relatively low temperatures in the range 177° C.; however, the coating surface energy does not appear to be low enough to provide efficient anti-stiction and passivation functions for MEMS. For example, T Mayer et al., in an article entitled: “Atomic-layer deposition of wear-resistant coatings for microelectromechanical devices”, Applied Physics Letters, v. 82 N17, 28 Apr., 2003, describe a thin, conformal, wear-resistant coating applied to a micromachines Si surface structure by atomic-layer deposition (ALD). Ten nm thick films of Al2O3 were applied to a silicon surface using a binary reaction sequence with precursors of trimethyl aluminum and water. Deposition was carried out in a viscous flow reactor at 1 Torr and 168° C., with N2 as a carrier gas, Cross-section transmission electron microscopy analysis showed that the films were uniform to within 5% on MEMS device structures having an aspect ratio ranging from 0 to greater than 100. The Al2O3 film produced was stoichiometric.
Preliminary friction and wear measurements for the 10 nm thick Al2O3 films showed a friction coefficient of 0.3 for a Si3N4 ball sliding on a flat Al2O3-coated substrate, and less wear particle generation than for a native-oxide-coated silicon substrate. At the time of publication of the paper, the nature of the wear and failure process as a function of applied load had not yet been determined.
There remains a need in the industry for a low energy coating which can provide efficient anti-stiction and passivation functions for mems and which can be produced at low temperatures which are more tolerable to various MEMS substrates.